In a high-frequency package mounted with a high-frequency semiconductor device, which operates in a high-frequency band such as a microwave band or a millimeter-wave band, in consideration of environmental resistance and operation stability of the high-frequency package, the high-frequency semiconductor device is often mounted in a cavity hermetically and electrically shielded by a cover, a seal ring, or a grounding conductor.
However, resonance occurs in a frequency band with a cavity dimension, which depends on a member such as the cover, about a half or an integer times as long as that of a free-space propagation wavelength. As a result, an operation of the semiconductor device in the cavity and a characteristic of a transmission line become unstable. Particularly, in a high-frequency semiconductor device that operates in a millimeter-wave band (30 gigahertz to 300 gigahertz), a dimension of the device and that of a propagation wavelength corresponding to a signal frequency come close to each other. Thus, it is difficult to make a dimension of the cavity for housing the device a half or less of the propagation wavelength corresponding to the signal frequency. Consequently, a high-order resonance mode tends to occur. In particular, in a millimeter-wave radar that operates in a 76-GHz band, a free-space propagation wavelength is about 4 millimeters in this frequency band and a size of a cavity necessary for mounting a plurality of high-frequency circuits with a size of 1 millimeter square to 3 millimeters square is about 10 millimeters. Thus, cavity resonance is likely to occur.
To control such cavity resonance, Patent Document 1 discloses a conventional technology for forming a radio wave absorbent and a resistor film in a region inside a cavity such as a rear surface of a cover.
Patent Document 1: Japanese Patent Application Laid-open No. H8-18310